Water, wastewater and liquid granular filtration units used by municipalities and large industrial plants typically employ a gravity filter system to produce useful water and liquids. These filters have an underdrain system that serves multiple purposes; the underdrain system supports and separates the filter media from the filter bottom, collects the water that passes through the media, and distributes backwash water and/or air across the filter.
Underdrain systems are frequently made of concrete blocks having spaces to allow for piping that is part of the backwash distribution system and sumps that collect the filtrate. One type of underdrain system commonly used in municipal filters is a monolithic false bottom underdrain system. Referring to FIG. 1, the monolithic bottom underdrain system is referred to as a false bottom system because it is comprised of inverted pyramidal or cone-shaped depressions cast into a concrete block 54 that overlays the structural floor or true bottom of the filter housing. At the inverted apex of each depression is a porcelain thimble (or nozzle) 55 leading to the collecting sump below the false bottom. The concrete underdrain system 52 is supported above the structural floor or true bottom of the filter by ledges provided in the filter walls and by supporting piers. Monolithic underdrain systems have used gravel support beds as media support systems. Media in gravity filters serve to filter the influent water for the removal of solid debris and flocculated particles. The media can be sand, coal or a combination of both. A media support system, such as the gravel bed 53 shown in FIG. 1, supports and separates the media from the underdrain system. When layered gravel beds are used, the bed of gravel is usually 12 to 18 inches in height with several layers of varying size gravel. Porcelain spheres 57 ranging in size from about one inch to about three inches are placed to fit snugly into the cone-shaped depressions. The porcelain spheres support the gravel layer above the depressions to allow for a backwash system. During the filtration process, debris and flocculated particles tend to clog the media and gravel layers and therefore must be removed periodically. Water backwash under high velocity is used to cleanse the media and gravel layers of this debris. Air backwash is also desirable because of the improved scouring properties of compressed air. Until the present invention, however, air backwash was not possible in gravity filters having monolithic underdrain systems.
Gravel layers used as a media support bed in monolithic underdrain systems have several disadvantages. A primary disadvantage is the uneven distribution of backwash resulting from jet turbulence patterns in water backwash. Other disadvantages of gravel layer support beds include difficulty in reinstallation, the need for deeper filter boxes to allow for the depth of the gravel and inability to use an air backwash or scouring with system, which enhances the cleansing process of the media. Also the gradation of the gravel layers tends to be disturbed during the filtration and backwashing processes and downtime may be required to restore the desired gradation.
A precast concrete, plastic-jacketed underdrain block is disclosed in U.S. Pat. No. 4,923,606 to Gresh et al. Nozzle-less type underdrain systems with large openings for the passage of the filtrate and the backwash water are preferred because they do not plug as easily as nozzle type underdrains. Because the openings in nozzle-less underdrains are larger than the size of the individual grains of the media, however, it is necessary to use a media support system between the underdrains and the media. U.S. Pat. Nos. 5,149,427 and 5,232,592 to Brown disclose a cap for filter underdrain blocks comprising a porous, planar body. The body of the cap is said to be adapted to support a fine grain filter media without the media penetrating therethrough. The pores in the cap body are approximately 700-800 microns in size.
U.S. Pat. No. 5,618,431 to Kondo et al. discloses a method of preparing and cleaning floating filter medium for biological filtering apparatus. The steps of the Kondo '431 method are: forming a filter layer of granular floating filter medium having a specific gravity of about 0.3 or less and a particle size of 1 to 15 mm; allowing wastewater to enter the treating tank and flow through the filter; and separating and removing excess sludge from the filter medium by discharging an amount of water equivalent to from 0.5 to 1.5 times the filling amount of the filter medium in a time of between 5 to 90 seconds. A method and apparatus for filtering water with reduced spillage is disclosed in U.S. Pat. No. 5,635,079 to Becking, II . A water filtration unit which is sealed with an upper and a lower plastic bottle is taught by the '079 patent.
Porous plates have been used to replace gravel layers. Porous plates are typically manufactured from sintered plastics. Plastic porous plates, however, are usually buoyant and need to be secured in some way to prevent lifting, especially during the backwash cycle. Prior art methods of securing the porous plate include a combination of screwing and caulking or grouting the plate to the underdrain blocks as disclosed in U.S. Pat. No. 5,149,427 to Brown, or bolting the plate to the underdrain blocks. Small irregularities in the floor of the filter, the underdrain blocks and the plates can cause seal failures between the plates. Seal failure allows media to penetrate the media support system, causes a progressive failure of the filter underdrain and then of the filter system itself. The underdrains, effluent piping, and clearwell may become plugged with media and the filter bottom may collapse due to excessive pressures which develop during backwash. Savage, in PCT International Publication WO 97/40907 entitled Fluid Treatment Media Support System, incorporated herein by reference, discloses a porous plate with multiple layers of fine sized and coarse sized pores and a system for anchoring the porous plate to air laterals below concrete block underdrain systems.
None of the above-referenced patents disclose or teach a method of retrofitting a monolithic false bottom underdrain system with a porous plate that supports the media, allows for an air distribution system that uniformly distributes air backwash, and securely anchors the porous plate to the filter bottom. Nor do they teach a method of filtration using a monolithic false bottom underdrain system retrofitted with a securely anchored porous plate that allows for both air and water backwash.
What is needed is a method for retrofitting gravity filters with a securely anchored filter media support system that uniformly distributes backwash throughout the media. A method for retrofitting gravity filters with a filter media support system that allows the utilization of an air backwash to further scrub the filter media would also be beneficial.